ARM: OMAP3: Add ADS7846 touchscreen support to Overo platform, v3

An upcoming Overo expansion board includes an ADS7846 touchscreen controller.

This patch adds support via the ads7846 driver when enabled in the
kernel config.

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>